The efficient operation of a vertical antenna for transmitting or receiving requires that the antenna be used in conjunction with a "ground". The type, size, physical construction and electrical characteristics of the choosen "ground" determine the performance, efficiency, ease of feed and radiation pattern of the antenna system.
Basically there are two requirements for a good "ground". First, the ground must provide a low resistance path for the return of those currents resulting from the radiated field of the antenna. This is of particular importance at the base of the antenna where the return currents are concentrated. Current flow through the relatively high ground resistance in this area can result in severe and substantial (I.sup.2 R) heat losses.
Secondly, a good ground system should provide a good electrically reflecting plane to allow the creation of the so called "image antenna". This increases the signal radiated by the antenna and assures that it is transmitted or received at the desired radiation angle.
Conventional vertical antennas that use the earth as a ground do not provide sufficient and optimum performance. This is because the earth is a relatively poor electrical conductor at low frequencies such as those used by AM broadcast stations and acts as a dielectric or insulator at high frequencies such as used by land mobile radio stations, citizens band radio service, and the higher amateur radio bands.
As opposed to using the earth as a sole "ground" source, it has been known to use artificial ground systems in conjunction with vertical antennas. There is no doubt, that artificial ground systems do improve the operational efficiency and performance of vertical antennas.
The most commonly used artificial ground system comprises a system of buried electrical wires that radiate outwardly from the base of a vertical antenna. Antenna return currents flow back towards the base of the antenna along these wires instead of flowing through the adjacent high resistance earth. Research conducted in the past, particularly that reported in 1937 by George H. Brown, determined that for optimum results one should use 120 or more radial wires with each being at least one-fourth wavelength long relative to the operating frequency of the antenna system. In fact, this very recommendation is a requirement of the Federal Communications Commission for AM broadcast stations today.
It is appreciated that buried wires provided over a substantial area of the earth can be very expensive and time consuming to install. In view of this, an artificial ground system referred to as a "counterpoise" has been occasionally used in the past. A counterpoise is an artificial ground system comprising a series of radial wires or other forms of electrical conducting material that is disposed above the earth as contrasted to being disposed below the earth.
Construction problems associated with providing counterpoises have precluded extensive use of the systems, particularly in low frequency applications. Prior to Applicant's invention and the research and development work associated therewith, there was little, if any, data or information concerning the electrical and performance characteristics and properties of the counterpoise. Because of the absence of this data, technology and information, it has been dificult for individuals to design a reliable counterpoise artificial ground system based on existing design criteria and perameters.
It should also be pointed out that the so called "ground screen" has also been occasionally used as an artificial ground system. The "ground screen" may be physically similar to a counterpoise, but is designed such that its wires are grounded to the earth. Ground screens are typically used in a fashion where the wires are buried until they reach the vicinity of the antenna at which time the wires extend up above ground level to form the ground screen. Again, as in the case with the counterpoise, there was very little, if any, prior information and literature available concerning electrical performance characteristics of the ground screen.